linux-kernel - Re: [RESEND PATCH v2] devres: Really align data field to unsigned long long

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Date:   Mon, 9 Jul 2018 09:07:13 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Greg KH <greg@...ah.com>
Cc:     Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        arcml <linux-snps-arc@...ts.infradead.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        stable <stable@...r.kernel.org>
Subject: Re: [RESEND PATCH v2] devres: Really align data field to unsigned
 long long

On Mon, Jul 9, 2018 at 7:49 AM Greg KH <greg@...ah.com> wrote:
> On Mon, Jul 09, 2018 at 07:44:44AM +0300, Alexey Brodkin wrote:
> > Depending on ABI "long long" type of a particular 32-bit CPU
> > might be aligned by either word (32-bits) or double word (64-bits).

Or even 16-bit (on e.g. m68k).

> > --- a/drivers/base/devres.c
> > +++ b/drivers/base/devres.c
> > @@ -24,8 +24,12 @@ struct devres_node {
> >
> >  struct devres {
> >       struct devres_node              node;
> > -     /* -- 3 pointers */
> > -     unsigned long long              data[]; /* guarantee ull alignment */
> > +     /*
> > +      * Depending on ABI "long long" type of a particular 32-bit CPU
> > +      * might be aligned by either word (32-bits) or double word (64-bits).

or even 16-bit (on e.g. m68k).

> > +      * Make sure "data" is really 64-bit aligned for any 32-bit CPU.
> > +      */
> > +     unsigned long long              data[] __aligned(sizeof(unsigned long long));
> >  };
>
> Does this change the padding today for any other arches?  If so, does
> the increased memory usage cause any noticable issues?

Yes it does. struct devres_node contains an odd number of pointers, so
there will be a hole between node and data on all 32-bit architectures.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds